The present invention relates to the field of calenders, and more particularly to devices for controlling the diameter of the rolls used in calenders or analagous machines.
Pressing a material between two calender rolls can change the physical characteristics of the material. For example, calendering paper changes its density, thickness and surface features. Thus, the calendering process is frequently used to manufacture paper and other sheet materials.
A common problem associated with calendering is the uneven thickness of the calendered material, or "web". Localized variations in the diameter of individual calender rolls creates variations in the spacing or "nip" formed between cooperating rolls. Variations in the nip across the width of a pair of calender rolls produces a web having non-uniform thickness. Therefore, a more uniform thickness can be attained by controlling the local diameter of the rolls.
If the rolls are made of a material that responds to changes in temperature by changing at least one dimension, one may control local roll diameters by varying the temperature of selected cylindrical sections of the calender roll. Previous devices have used this principle by directing jets of hot or cold air against sections of a rotating calender roll to control its local diameters.
Many of these devices blow jets of hot air from a supply plenum against selected sections of the calender roll to increase its local diameter and thus decrease the local thickness of the web. Alternatively, when these devices blow jets of cold air from a separate supply plenum against selected cylindrical sections of the calender roll, the adjacent sections contract. This decreases the local roll diameter and increases the local thickness of the web.
Nozzles communicating with the interior of each plenum direct these jets of air against the calender roll. The nozzles are disposed at intervals corresponding to adjacent sections of the calender roll whose local diameter is to be controlled. Examples of such devices are shown in U.S. Pat. No. 2,981,175 to Goyette, U.S. Pat. No. 3,177,799 to Justice and U.S. Pat. No. 3,770,578 to Spurrell.
Valves have often been used to control the flow of air through each nozzle. Where separate plenums provide the hot air and cold air, many such devices require two valves and two nozzles to control the diameter of each section of the calender roll. Alternatively, a dual control mechanism may be used to mix the relative volumes of hot and cold air from the two plenums and then release the air through a single nozzle. In either configuration, this redundancy can increase the cost of of these devices.
Another problem experienced with controllers of this type is that accurate control of the roll diameter can require precise metering of the air jets. Therefore, the valve control mechanisms generally should not exhibit hysteresis effects so that they can obtain repeatable settings regardless of whether the valve is being opened or closed. Furthermore, these control mechanisms usually must be capable of operating at high or low temperatures. However, even when the valves work properly and the control mechanisms accurately control the size of the valve orifices, the rate that air is released through the nozzles is often variable because the air pressure in each plenum depends upon both the number of valves open at one time and the volume of air released through each nozzle. Thus, the flow of air through the nozzles in these devices can be difficult to control.
These devices are also subject to other limitations and inefficiencies. For example, the nip control range is a function of the maximum and minimum temperatures of the air jets. However, the hot air in the plenum is typically heated by waste steam from the facility power plant. Steam supplied by such a power plant usually has a maximum temperature of about 350.degree. F., and inefficiencies in the heat exchange process further limit the maximum temperature of such steam heated air to about 325.degree. F.
Furthermore, to maintain the air temperature at 325.degree. F., hot air must be continuously supplied to the hot air plenum, even when hot air is not being released through the nozzles. If hot air is not continuously supplied to the hot air plenum, the stagnant air in the plenum may cool to ambient temperature. Then, when a jet of hot air is required to increase the diameter of a section of the calender roll, the cooled stagnant air must first be purged from the plenum. This increases the response time of the device.
The calender roll control device of the present invention has a number of features which overcome many of the disadvantages of calender roll control devices heretofore known. It can provide a constant flow of air from a single plenum and it can accurately adjust the temperature of a plurality of air jets. Since it requires only one plenum and can operate without flow control mechanisms, the device has a relatively low initial cost. Additionally, it does not require steam heating equipment. Instead, the device heats the air jets only where and when necessary to increase the roll diameter. Furthermore, because it produces hotter air jets than are typically provided by steam powered equipment, the device of the present invention can provide more than twice the nip control range on a typical 12" diameter 190.degree. F. calender roll. These and other advantages will become apparent in the description which follows.